The present invention relates to an improvement in a method for the seam welding of the side seam or overlapped portion of a tubular form, for example, a cylindrical, square or elliptical tubular form, which is welded to be formed into a can body, using an automatic electric resistance seam welder (hereunder referred to only as a welder).
Welding using the above type welder is well known, but, to make sure, we will explain it with reference to FIG. 1.
First, pieces of thin metal plate cut to predetermined dimensions are pulled out from their stacking station 1 one by one to make tubular forms surrounding a horizontal supporting arm 2 (in FIG. 1 one of them is shown as a cylindrical tubular form A represented by a two-dotted chain line). The tubular forms move along this supporting arm 2 from the left to the right in the drawing. A lower electrode wire guide part 3 is positioned at the right end of the supporting arm. The lower electrode wire guide part 3 consists of a horizontal fixed axle 4 which extends perpendicularly to the horizontal supporting arm 2 and is fixed to arm 2, and a lower electrode wire-supporting roll 5 which is rotatably fitted around said axle 4. Just above the lower electrode wire guide part 3, there is positioned an upper electrode wire guide part 12 which comprises a horizontal revolving axle 11 which is parallel to the horizontal fixed axle 4 and an upper electrode wire-supporting roll 10 which is fitted over axle 11 and revolves together with axis 11. A lower electrode wire 6 runs along the underside of the horizontal supporting arm 2 from the left to the right in the drawing to go around the circumferential surface of the supporting roll 5 and then goes back to the left in the drawing again along the underside of the supporting arm 2. An upper electrode wire 14 runs along a guide roll 13 and the circumferential surface of the upper electrode wire-supporting roll 10 and goes back to the left in the drawing. The upper electrode wire 14 is moved in the same direction as said roll 13 and in a counterclockwise manner by supporting roll 10, whereas the lower electrode wire 6 is moved by being coiled round a reel (not shown), and the lower electrode wire-supporting roll 5 is revolved in a clockwise manner by such movement.
Each of the above tubular forms has an overlapped portion 20 which is formed by the two edge portions of a piece of thin metal plate and moves with said overlapped portion 20 being at the top. The overlapped portion is inserted between the upper and the lower electrode wires 14 and 6. Push pressure is applied downwards to the upper electrode wire guide part 12, so that overlapped portion 20 is pressed between the two electrode wires. Thus, the overlapped portion moves as the two electrode wires progress. During this movement welding current passes through the upper electrode wire guide part 12, the upper electrode wire 14, the overlapped portion of a tubular form 20, the lower electrode wire 6 and the lower electrode wire guide part 3. The overlapped portion is melted and subjected to pressure welding as the facing surfaces of the overlapped portion of a piece of thin metal plate and its vicinity is heated by the contact resistance of the facing surfaces. Thus, the overlapped portion is subjected to what is called electric resistance seam welding.
In this case, as is shown in FIG. 1, a preceding cylindrical tubular form A.sub.1 is in a position just before its release from the contact with the upper and the lower electrode wires 14 and 6, the directly following cylindrical tubular form A.sub.2 is in a position to have just entered the contact with the upper and the lower electrode wires, and thus both the rear end 21 of the preceding cylindrical tubular form and the front end 22 of the following cylindrical tubular form are located between the upper and the lower electrode wires. If the following cylindrical tubular form is inserted between the upper and the lower electrode wires after the preceding cylindrical tubular form has completely passed through between the wires, a higher push pressure is applied to the edge part of the overlapped portion of each of the tubular forms, and, as is shown in FIG. 2, projections 25 and 26 which run from the cylindrical tubular form and extend longitudinally along the overlapped portion 20 are formed at the edges of the overlapped portion 20. Therefore, the above position shown in FIG. 1 is a countermeasure to prevent this formation.
Can bodies have already been manufactured by subjecting the overlapped portions of cylindrical tubular forms made of thin tinplate having a thickness of 0.2-0.4 mm to seam welding according to the above-mentioned method. In the process of producing can bodies, the current supplied to portions to be welded changes within a certain range during the continuous manufacture over a long period of time, depending on the change of the appliances in temperature and so on. When the welding is made by setting the current at such a value that both edge parts of the overlapped portion with a predetermined weld strength can be prepared by even the minimum current within the range of fluctuation, a can body A welded when the current has increased to a higher value within the above range of fluctuation under continuous manufacture over a long period of time, has projections 25 and 26 which run from both edges 23 and 24 of the can body A.sub.0 and are on the extension line of the overlapped portion 20 as is shown in FIG. 2. Such projections 25 and 26 are undesirable because they become a cause of damage to the seal between a can cover and a can body unless they are extremely small. Such being the case, as a countermeasure to prevent their formation, the welding has been made by applying a lower weld current to both edge parts 27 and 28 of the overlapped portion 20 than to the center 29 or the parts other than the edge parts of the overlapped portion so that projections cannot be formed even if the welding is made using the maximum current within the above range of fluctuation. However, the current applied to both edges parts of the overlapped portion becomes lower than the minimum current value for both edge parts to get a requisite weld strength when the current value decreases to a lower value within the range of fluctuation, and in this case the resulting weld strength of both edges parts is poor, and does not match the predetermined strength. Can bodies which have both edge parts of weld strength lower than the predetermined strength suffer from separation or breakage (hereunder referred to as crack) of both edge parts of the overlapped portion when flanges extending radially and outwardly from the can body are formed on both opening end portions of the can body. Can bodies having these cracks are unacceptable because the seal between such can bodies and the can covers to be seamed on them would be poor.
Then, as a further countermeasure, we, the inventors of the present invention, tried to control the change in supply current manually while watching the change with a current meter thereby controlling the fluctuation within a narrow range and to weld the overlapped portion of each of the tubular forms by current within such narrow range without making the weld current applied to the both edge parts lower than that applied to the center --that is--by the weld current of the same value along the overall length of it. As a result, we have found that, in spite of changing of the value of the current within the narrow range during the continuous production over a long period of time, a satisfactory effect is obtained. That is, no substantial formation of projections extending from both edges of the overlapped portion mentioned above was observed even by the weld using the maximum current within the narrow range, while the minimum current within the range provided both edge parts with satisfactory weld strength and did not cause a crack at the edge parts when the flanges were formed. For this reason, tinplate-made can bodies can now be welded and manufactured according to this method.
We have further tried to subject a chromate-treated steel plate-made cylindrical tubular form to electric resistance seam welding by the utilization of the above welding method. First, when the whole of the overlapped portion was welded with the lowest current which can provide both of the edge parts with the requisite weld strength, as is shown in FIGS. 3 and 4, many short thread-like or hair-shaped solid pieces 32 (hereunder referred to as burrs 32), which seem to be a result of the scattering or extrusion of the softened or melted parts of the overlapped portion, were formed extending outwardly from a longitudinal edge 31 of the inner side of the overlapped portion 20 (the inner face side of the tubular form). These burrs increased in number and size as the supply current became higher. If these burrs 32 are large, they become a bar to the application of a paint for coating the inner surface of the welded overlapped portion. If a paint coating cannot be satifactorily applied, cans comprising such can bodies as have such burrs tend to damage the quality of the contents. Accordingly, the best condition of the can bodies is that they have no burrs. Even if burrs are formed, in order that the can bodies be satisfactory for use, the burrs must be so small that they do not prevent the application of paint and are well coated with the paint (hereunder this condition of the can bodies is referred to as the condition where there is no substantial formation of burrs). Therefore, next, the supply current was lowered to a value which did not cause the formation of burrs but was sufficient to weld the whole of the overlapped portion. As a result, it has been found that the manufactured can bodies comprise a substantial number of can bodies having both of the edge parts insufficient in the weld strength.
Further, we have found that can bodies having no burrs and having the predetermined weld strength can be manufactured if the change in the supply current is substantially nil. However, the equipment necessarily becomes highly expensive in order to make the change in the supply current substantially nil, and thus it is not practical from an industrial viewpoint to so control the supply current.
Surely, there has been suggested a method for the welding where the chrome layer on the overlapped portion has been removed prior to the welding. But, this method also is not practical because in a high speed production line where several hundreds of cans are manufactured per minute it is extremely difficult to effect the removing operation at a high speed, accurately, continuously and over a long period of time, by any known physical and chemical means.
Furthermore, among the prior art methods, the welding method where the current applied to both of the edge parts of the overlapped portion is lower, cannot provide can bodies of good quality even if the can bodies are manufactured by welding steel plate having a chrome coating on the surface.
Accordingly, it is an actual aspect of the art that can bodies are not mass produced by welding steel plate having a chrome-coating such as chrome-treated steel plate.